The present invention relates to a chain tensioner for an internal-combustion (IC) engine having a hydraulic piston which is longitudinally guided in a housing bore of a stationary housing and, on one side, is equipped with a tensioning shoe resting against a drive such as a chain or belt, and, on the other side, is loaded by a coil spring supported on the housing.
A chain tensioner described in German DE-PS 31 45 115 has a hollow piston longitudinally guided in a housing bore of a hydraulic housing. A tensioning shoe is placed on the top of the hydraulic piston. When the hollow piston is acted upon on its bottom side by hydraulic pressure, the chain is tensioned by the tensioning shoe. The hydraulic fluid is fed into the hollow piston via a ball check valve. When the hydraulic piston is loaded by chain thrashing, the hydraulic piston has the effect of a rigid body because the hydraulic fluid is almost incompressible, and the check valve prevents fluid from escaping. An excessive positive pressure is prevented by the opening of a relief valve. In addition to being acted upon by pressure, the hydraulic piston is acted upon by a coil spring on its bottom side.
It is an object of the present invention to provide a chain tensioner such that, when loaded by an impact, the hydraulic piston has higher flexibility and prevents a swinging and thrashing of the drive more effectively.
The foregoing object has been achieved in accordance with the present invention by using only a coil spring for tensioning the drive, e.g. a chain, with the hydraulic pressure controlling a hydraulic piston from both sides such that the piston is damped in both moving directions. The hydraulic piston is mechanically braced by a compression spring, and the hydraulic fluid acting upon it can escape when the chain is loaded by jolts. Thus, an excellent elastically damped chain tensioning is implemented.
Another advantage of the present invention is higher operational reliability. Conventional hydraulic chain tensioners are not fully operative during engine start if the hydraulic pressure has fallen after a prolonged stoppage of the internal-combustion engine and must first build up again after the start. Since the chain tensioner according to the present invention is tensioned mechanically, it is fully operative also during the start.